Question about B-Frames and IQ

[Chainmax]

Is there any way to use B-Frames without any quality loss at all (even at high bitrates)? Maybe using settings like x/100/x/x? If not, why do B-Frams imply a slight loss in IQ and P-Frames don't?

[Sharktooth]

Use the (in)famous search button:P

[Chainmax]

I don't know what to look for. If I look for "b-frames" I'll have to wade through dozens of threads that probably won't have anything to do with what I'm looking for.

[mf]

Originally posted by Chainmax
I don't know what to look for. If I look for "b-frames" I'll have to wade through dozens of threads that probably won't have anything to do with what I'm looking for.
searching for b-frames highest quality gave me this thread (http://forum.doom9.org/showthread.php?s=&threadid=48443&highlight=bframes+highest+quality) :rolleyes:.

[Tommy Carrot]

Originally posted by Chainmax
Is there any way to use B-Frames without any quality loss at all (even at high bitrates)? Maybe using settings like x/100/x/x? If not, why do B-Frams imply a slight loss in IQ and P-Frames don't?

Your last assume is not exactly true. P-frames also degrade the quality, otherwise it would be lossless, isn't it. :D

Seriously, the idea behind the b-frames is, they are more compressed than the p-frames, but because no other frames depend on them, they would degrade the overall quality less than a similarly compressed p-frame, which is used as base for another p-frame. So the bitrate gain is higher than the quality loss.

[Chainmax]

Originally posted by Tommy Carrot:
Your last assume is not exactly true. P-frames also degrade the quality, otherwise it would be lossless, isn't it. :)

Seriously, the idea behind the b-frames is, they are more compressed than the p-frames, but because no other frames depend on them, they would degrade the overall quality less than a similarly compressed p-frame, which is used as base for another p-frame. So the bitrate gain is higher than the quality loss.
Remember CorePNG, it implemented P-frame support while still being lossless. I understand how partial frames (P and B) can be compressed further than P-frames and have almost unnoticeable effects on IQ, but that doesn't mean that P and B-frames should always have higher compression. Just having them on a stream with the same compression ratio as a regular keyframe will also yield good final sizes.
Since P-Frames don't have a settings window in XviD I don't know if they are more compressed than keyframes (the max-min quantizer window doesn't count IMO). Can someone confirm/deny if they are or not?

Basically, what I want to know is exactly what settings would imply that B-frames don't get any extra compression from the one used on P and I-frames. While we're at it, I'd also like to know why the B-Frame quantizer was set to be determined from the averaging if the quantizers of its nearest non b-frames.

mf, thanks for the linkage. Still, I don't understand why a quantizer ratio of 100 means b-frames are compressed with the same quant as the rest of the movie. Let's say that the nearest no b-frames have quants 4 and 8, then the b-frame would be compressed at quant (8+4)/2 = 6, and 6<>4 or 8....